The present invention relates to a printer and, more particularly to, a stencil printer for printing an image represented by perforations formed in a master on a paper or similar recording medium.
It is a common practice with a stencil printer to wrap a master around an ink drum and feed ink to the master via an ink feeding means. A press roller, press drum or similar pressing means is pressed against the ink drum at a position where the pressing means faces the ink feeding means, so that an image is printed on a paper at a print section where the ink drum and pressing means face each other. The paper with the image, i.e., a printing is conveyed to a paper discharge section by a belt while being retained on the belt by a suction fan. Separating means is arranged above the belt and includes a peeler for peeling the paper adhered to the ink drum due to the viscosity of the ink, and an air knife for sending a stream of air from the edge of the peeler in order to promote the separation of the paper from the drum.
However, the above stencil printer has the following problems left unsolved. When the paper carries a solid image at its leading edge portion or when the image ratio at the leading edge of the paper is great, adhesion between the paper and the ink drum increases and prevents the paper from being adequately peeled off from the drum. As a result, the paper tends to roll up and has its image surface smeared by the peeler in the form of marks. Further, the air knife and suction fan associated with the peeler and belt, respectively, produce noise due to the stream of air and suction.
In light of the above, Japanese Patent Laid-Open Publication No. 9-24604 teaches that a charged belt is positioned downstream of a print section where pressing means and an ink drum face each other in the direction of paper conveyance. The belt causes a paper to electrostatically adhere thereto and thereby separates it from the ink drum. The above document also teaches that a belt is passed over a roller facing a print drum and a roller located downstream of the drum in the direction of paper conveyance. In this configuration, the belt is angularly movable about the downstream roller into and out of contact with the ink drum; a position where the belt and drum face each other define a print section.
A problem with the above angularly movable belt scheme is that a space broad enough for the belt to be bodily angularly moved about the downstream roller relative to the ink drum is necessary below the ink drum and increases the overall size of the printer. Another problem is that an exclusive mechanism for moving the entire belt into and out of contact with the ink drum is required, sophisticating the construction of the printer. Particularly, when the belt is arranged downstream of the print section defined by the pressing means and ink drum, the mechanism for so moving the belt must be provided independently of a mechanism for moving the pressing means. A further problem is that because the belt is passed over the roller facing the ink drum and the downstream roller, the conveying surface of the belt cannot cover the portion upstream of the print section. As a result, the conveyance of the paper to the print section and the entry of the same into the print section are irregular, rendering the position of an image on the paper unstable.
On the other hand, Japanese Patent Laid-Open Publication No. 1-290489 discloses a stencil printer with a multicolor printing capability and including a plurality of ink drums arranged side by side in the direction of paper conveyance. The ink drums each is supplied with ink of particular color. Particular pressing means is pressed against each ink drum with the intermediary of a belt, causing the drum and belt to nip a paper for printing an image thereon. After an image has been printed on the paper by the upstream ink drum, the paper is conveyed toward the downstream drum while being electrostatically adhered to the belt.
The multicolor stencil printer taught in the above document has some drawbacks, as follows. Assume that a solid image is printed on the leading edge portion of a paper, that the image ratio of the leading edge portion of the paper is great, or that the viscosity of ink is caused to vary by the varying ambient temperature. Then, the paper cannot be adequately peeled off from the upstream ink drum and rolls up. That is, the belt with the suction fan cannot sufficiently suck the paper thereonto, depending on the condition of the image and/or the viscosity of the ink. As a result, the timing for the paper to reach the downstream ink drum is delayed. The delay renders the timing for ink to be transferred from the downstream ink drum to the paper irregular, resulting in the dislocation of an image or the overlapping of image components of different colors. Although the sucking force of the suction fan may be intensified in order to prevent the paper from rolling up, such an approach would aggravate noise ascribable to suction. Thus, there is an increasing demand for a new mechanism capable of conveying a paper while surely retaining it.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 60-148864, 60-148866, 4-133083, 4-322277, and 9-104158.